Byoung Kwon Choi

Influence of ammonia on the corrosion behavior of Ti−Al−Zr alloy in 360°C water

The influence of ammonia on the corrosion behavior of Ti−Al−Zr alloy was evaluated in pressurized water at 360°C for 600 days. The results of the corrosion test indicated that the addition of ammonia accelerated the corrosion and hydrogen pickup rates. The oxide scales were composed of a double layer structure, and an inner layer was covered by an outer layer of oxide grains. The XRD studies on the oxide scales showed that the anatase-TiO2 was continuously transformed into rutile-TiO2 as the corrosion proceeded. When the same weight gain was considered, the oxide layer grown in water appeared to have a much higher rutile weight fraction than that grown in the ammonia aqueous solution. The deterioration of the corrosion behavior by the ammonia could be mainly attributed to the retardation in the transformation of the oxide structure from a metastable anatase phase to a stable rutile one.

Low Cycle Fatigue Behavior of Zirconium Tube under the Cyclic Pressurization

Study was attempted to produce stress-life curve of the zirconium tube under the cyclic pressurization and to analyze its behavior. The internal pressurization machine was devised and the low cycle fatigue test at the zirconium cladding tube was carried out. The result showed that the O’Donnell and Langer relationship can be fitted to the fatigue behavior of the zirconium cladding under the cyclic pressurization, where the fatigue limit of the Zircaloy-4 at 350oC was shown at 341.6MPa. From the analysis of the diametral changes and the fractographic observation, a combined creep-fatigue interaction rather than a pure fatigue had an influence on the failure of the zirconium cladding under the cyclic pressurization at 350oC.

Effect of the Intermediate Cooling Process on the Behavior of the Zircaloy-4 Cladding After a High-Temperature Oxidation

Abstract Studies were conducted to investigate the effect of the intermediate cooling process on the thermal shock behavior of Zircaloy-4 fuel cladding under a simulated loss-of-coolant accident condition and to analyze the related mechanical and microstructural properties. The Zircaloy-4 specimen was oxidized at the desired temperature and time, then various cooling processes were applied such as the direct water quench, the intermediate cooling at 700°C for 200 and 2000 s, and the successive cooling from 950 to 700°C. The results showed that the direct water quenching without any intermediate cooling process reduced the cladding ductility in that it reduced the minimum equivalent cladding reacted from 20 to near 17%. Ring compression ductility decreased, and the minimum thickness of the prior-beta layer thickness that causes brittle failure increased from 0.3 to 0.4 mm in the case of the direct water quench condition. As the cooling rate increased, the size of the plate inside the prior-beta phase decreased so that it induced an increase in the residual dislocation density to result in a decrease of the cladding ductility. Additional oxidation effect during a slow cooling below 950°C had little influence on the cladding behavior.